Catheters, guidewires, stylets, and needles are used in most areas of medicine, particularly in cardiology, radiology, and surgery, to monitor various physical, physiologic and hemodynamic parameters such as pressure, flow, velocity, vessel caliber, pH, pO.sub.2, pCO.sub.2, and temperature. Catheters and guidewires can also function as tools to perform diagnostic imaging (angiograms) and to deliver treatment (angioplasty, physical and electrical ablation of lesions). Existing technology requires these catheters and guidewires to be directly connected by a fluid column or wired to external sensing equipment to measure most of these various physiologic parameters.
Catheters and guidewires can be used for diagnostic and treatment modalities within the urogenital system, which includes the bladder, ureters, urethra and the kidneys. When obstructive processes involve these areas, they can endanger kidney function. Similarly, pancreatic-biliary ductal systems can become obstructed by stones, strictures or tumors. In the gastrointestinal tract, the catheters and guidewires can be passed orally or transrectally and measure such parameters as pH, temperature, or hemoglobin. Monitors placed in the cervical canal can be used to assess uterine contractions, intrauterine pressure, fetal heart rate, pO.sub.2 and pH. Pressure monitors can also be placed at the end of small catheters or guidewires located in the tracheobronchial tree to monitor peak end-expiratory airway pressures of patients requiring mechanical ventilation. Current technology only allows for indirect measurement of this value at the level of the ventilator itself. A catheter tipped with a pressure sensor for directly monitoring peak end-expiratory pressures is desired for allowing physicians to better modulate ventilatory therapy.
Existing clinical cardiorespiratory technology uses fluid-filled central venous and balloon tipped pulmonary artery catheters to measure pulmonary artery pressure, central venous pressure, pulmonary capillary wedge pressure, temperature, and oxygen saturation. Cardiac output and systemic vascular resistance can then be derived. These pulmonary artery catheters are referred to as Swan-Ganz Catheters (or balloon-tipped, flow-directed catheters). Limitations of these catheters are described in detail above. Typically, pressure, temperature and pulmonary artery hemoglobin oxygen saturation are monitored directly while pO.sub.2, pH, pCO.sub.2 are measured in the laboratory from a blood sample drawn from the catheter.
Current technology also has limitations imposed by the properties of fluid within the catheter; the distance to the external transducer; properties of the material used in catheter construction, and size constraints which can lead to distortion and damping of the signal and creation of artifacts limiting precision and response frequency. Medical engineering's ability to diminish catheter size is currently limited by either wiring and/or open fluid column requirements. In some instances, patient mobility and transport are currently restricted by requirement for connection to an external monitor. Precise alignment of the catheter tip and the external sensing equipment is required with open fluid column-containing catheters to eliminate gravitational effects on hydrostatic pressure. The pH, pCO.sub.2, and pO.sub.2 of body fluids are usually determined by removal of blood from the external opening of the catheter; a slow, cumbersome process with risk for catheter occlusion via blood clotting and for blood-borne pathogen exposure. The removal of blood, and the requirement to discard a portion of the blood, often leads to an anemia in patients undergoing extensive monitoring or interventional procedures.
A common diagnostic and therapeutic procedure for vascular disease is an angiogram followed by an angioplasty. An angiogram is obtained with fluoroscopic imaging when radio-opaque contrast is injected into the artery through the open, hollow catheter. Once the target lesion is identified, the diagnostic catheter is exchanged over a guidewire for the balloon angioplasty catheter. The guidewire is then passed across the stenotic lesion. Once the wire crosses the lesion, a balloon angioplasty catheter is passed coaxially over the wire and placed at the site of the stenosis. The lesion is then dilated by inflating the balloon to open the channel. The pressure measurement through this open hollow catheter is virtually impossible with the wire in place. However, removal of the wire will cause loss of position across the "treated" lesion where a pressure gradient may still exist, requiring further angioplasty. Repeated insertion of guidewires and balloon-tipped catheters across vascular lesions may create atheroemboli, endothelial damage, thrombosis of the vessel, and dissection of the arterial wall.